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  #21  
Old 03-01-2014
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Re: Alternative to Solar

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Originally Posted by bobmcgov View Post
Nope. Teh box does not care how heavy or light the item it is attached to is. The controlling momentum resides in the weight (piston, pendulum, whatever) inside the magic box. What matters is acceleration, which is the rate of change of velocity, which itself is a combination of speed (a scalar) and direction. So the more an item changes speeds and/or direction, the greater its acceleration. The masthead and the keel bulb have the same period; that is, they complete one cycle of pitch or roll in the same length of time. But the top of the mast experiences far greater acceleration.

Now, take that acceleration and multiply it by the mass of the pendulum in the gizmo, and you have a force (F=ma). Multiply that force by the distance the gizmo's weight can shift within the bounds of its box (called 'stroke'), and you have F x d=Work; divide that by time, you have power. Multiply that power by time -- usually a different unit of time than time used to measure power itself -- and you have energy, like amp hour or watt-hour, which is what you store in yo' batteries. So a typical scenario (ignoring dissipative losses) for the OP's magic box might be:

Battery Juice = ((m*a)*d/t1)*t2 = ((15kg*4.9m/sec^2)*0.3m/2sec) = 11 Watts; 11 Watts*24 hours/day = 264 Watt-hours, or 0.26 kWh/day.

That's about what you would see from one 50W solar panel operating 5 hours a day.

Isn't physics fun?
Absolutely a great, informative post, thank you.
May I contact you if I have a physics problem in the future?
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  #22  
Old 03-01-2014
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Re: Alternative to Solar

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Originally Posted by bobmcgov View Post
Nope. Teh box does not care how heavy or light the item it is attached to is. The controlling momentum resides in the weight (piston, pendulum, whatever) inside the magic box. What matters is acceleration, which is the rate of change of velocity, which itself is a combination of speed (a scalar) and direction. So the more an item changes speeds and/or direction, the greater its acceleration. The masthead and the keel bulb have the same period; that is, they complete one cycle of pitch or roll in the same length of time. But the top of the mast experiences far greater acceleration.
Great post, helped me understand some things. Thank you. But attaching a box to the top of the mast or to the bottom of the keel also changes things in this example because it changes the mass distribution and movement. If we attach it to the top of the mast we will get more wild movement (good for the box, bad for the boat), and if we attach it to the bottom of the keel we will get less movement (good for the boat, bad for the box). If the amount of electricity generated by the gizmo depends on the mass of it's pendulum and acceleration, you can maximize mass of the box by adding it to the keel where that mass will not hurt the boat, or maximize acceleration (keeping the mass to a minimum) by adding it to the top of the mast. Honestly, I would much prefer to have a lot more mass at the bottom of my keel than more movement at the top of the mast - and that does not even begin to address mast strength/integrity issues with that extra weight on top.
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  #23  
Old 03-01-2014
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Re: Alternative to Solar

What if it was possible to move the box? You could hoist it up the mast when at anchor, so that even in a quiet harbor it got the most movement. Under way it could go on a bunk up forward, or in the anchor locker.
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  #24  
Old 03-01-2014
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Re: Alternative to Solar

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Originally Posted by krisscross View Post
If we attach it to the top of the mast we will get more wild movement (good for the box, bad for the boat), and if we attach it to the bottom of the keel we will get less movement (good for the boat, bad for the box). If the amount of electricity generated by the gizmo depends on the mass of it's pendulum and acceleration, you can maximize mass of the box by adding it to the keel where that mass will not hurt the boat, or maximize acceleration (keeping the mass to a minimum) by adding it to the top of the mast. Honestly, I would much prefer to have a lot more mass at the bottom of my keel than more movement at the top of the mast - and that does not even begin to address mast strength/integrity issues with that extra weight on top.
Good points. Those are the tradeoffs in play here. Matching the mass & stroke of the box's pendulum to the local movement of the boat attachment point would go a long way toward efficiency, and a heavier weight down low is certainly preferred to a lighter weight aloft, sailboat-wise. You start tripping over your own feet at some point, tho: a very heavy pendulum will be large in size, which will limit its room to move (stroke) within a box of given dimensions (tho mass increases as a cube while surface area squares). Depleted uranium is nice.

I've long thought incorporating AGM batteries into the keel would likewise be handy: as long as you are schlepping all that lead around, let's put it down low and have it serve two functions. Charging systems work soooo much better with large batteries, but on most boats there isn't enuf room for 1500# of batts, or you carry them higher than ideal. But of course, you'd need to be able to pull them out of the keel & replace them easily, and keep bilge water away from the connections, and so on.
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  #25  
Old 03-01-2014
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Re: Alternative to Solar

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Originally Posted by bobmcgov View Post
Charging systems work soooo much better with large batteries, but on most boats there isn't enuf room for 1500# of batts, or you carry them higher than ideal. But of course, you'd need to be able to pull them out of the keel & replace them easily, and keep bilge water away from the connections, and so on.
No question about it - juice storage capacity is just as important as charging ability. Seems like most sailboats are not designed to carry a lot of batteries weight where it is most beneficial (and that includes bilge design). It would probably be easier to design a boat taking it all into consideration - maybe even incorporating an electric motor that also has a backup gas or diesel driven generator like some cars these days. Would be a fun project to work on.
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  #26  
Old 03-01-2014
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Re: Alternative to Solar

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Originally Posted by bobmcgov View Post
Nah -- I'll defer to the greater physicist, which judging from your avatar, would be you. I'm just an English major. And let's remember, the example numbers above completely ignore dissipative losses and inefficiencies, which occur every step of the way. Most solar panels only convert around 15% of the solar energy that strikes them into electrical power. A quality wind turbine might reap 24% of the kinetic energy of the wind that passes thru its swept area. (Don't even ask what a coal-fired power plant loses along the way.) So let's say the OP's magic box is 20% efficient at converting kinetic energy of wave motion (at the box's location) into power at the output leads. I'd be astounded if it were that good, but let's pretend. Applying our 20% efficiency to the results above gives us 53 Watt-hours per day, or 0.05kWh. That's assuming pretty constant acceleration of 0.5g, which is the equivalent of a 200lb person experiencing a 100lb shove in the ribcage. Boisterous sea, in other words.
This effectively removes the benefit for nearly all cruisers. Such a device would generate 1.6Wh per pound, each day. My solar panels give me 2,800Wh on 5 hours of sunshine. And since I cruise in the summer and have an MPPT controller, I'm getting a more than 5 hours of sunshine. But even with that conservativeness, the device would have to weigh 1,700 pounds.

Unless the device is your keel or battery bank, or you are operating in a wavy place where the sun doesn't shine and the wind doesn't blow, there's no benefit. And that describes the opposite of good sailing.

I love innovation though, and there's always a place for such a device, somewhere.

Regards,
Brad
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  #27  
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Re: Alternative to Solar

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Originally Posted by bobmcgov View Post
...I've long thought incorporating AGM batteries into the keel would likewise be handy: as long as you are schlepping all that lead around, let's put it down low and have it serve two functions. Charging systems work soooo much better with large batteries, but on most boats there isn't enuf room for 1500# of batts, or you carry them higher than ideal. But of course, you'd need to be able to pull them out of the keel & replace them easily, and keep bilge water away from the connections, and so on.
Yes, ideally a bulb keel. Have thought about that too -- although not using AGMs. The keel would have to be a bit bigger, since the density of pure lead is higher, but you could use a good streamlined shape. It would mean more wetted area, but not so much as other, older keel designs.

Regards,
Brad
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  #28  
Old 03-01-2014
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Re: Alternative to Solar

Why not just try in incorporate one of these on the cockpit somewhere (stowing it when not in use) and have a good workout as well as charging up the batteries?


Last edited by guitarguy56; 03-01-2014 at 04:45 PM.
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  #29  
Old 03-01-2014
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Re: Alternative to Solar

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Originally Posted by Bene505 View Post
Yes, ideally a bulb keel. Have thought about that too -- although not using AGMs. The keel would have to be a bit bigger, since the density of pure lead is higher, but you could use a good streamlined shape. It would mean more wetted area, but not so much as other, older keel designs.

Regards,
Brad
A large keel battery would work till you hit a rock or submerged structure... I imagine that would kick off many alarms.
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  #30  
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Re: Alternative to Solar

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Why not just try in incorporate one of these on the cockpit somewhere (stowing it when not in use) and have a good workout as well as charging up the batteries?

That's another RE idea that pops up from time to time: human-powered electrical generation. It's not too bad in some ways, but its primary value is illustrating (in ways we can comprehend) just how freakin' difficult it is to make meaningful amounts of electrical power. A fairly fit athlete -- and most cruisers are not all that -- can produce 1/4 to 1/3 horsepower during vigorous exercise: bicycling, rowing, x-country skiing. Maybe half a horsepower in short bursts, but figure a quarter to a third ongoing.

One horsepower is 746 watts. Pedaling or rowing like the devil, with an efficient direct-drive PM alternator, will produce ~200W. That's real electricity for sure, but it also means you'd need to hold that pace for five hours to generate as much power (1 kWh) as 200W of solar panels on a medium-sunny day. Your refrigeration may use that much juice in a warm climate. It's good exercise, mind you; and in the RE world, some Watts is better than no Watts, as long as the means of generating them don't excessively ruin other aspects of the boat, like weight distribution, windage, stowage room, or cruising budget.

Speaking or ruining the budget ... a Watt&Sea tow generator will produce that same 1 kWh in about four hours sailing at 6kts. It also only works while under way. And a high-output aux generator or engine alternator will take just about an hour to bank the same. No surprise that, faced with depleted batteries and set of bicycle pedals, most sailors would reach for the ignition key.
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